Conventionally, electrolysis of a salt solution has been performed widely for the purpose of producing chlorine, hydrogen, and caustic soda. Chlorine generated by electrolysis is used in the production of sodium hypochlorite, hydrochloric acid, and liquid chlorine. Since common salt used as a raw material in electrolysis usually contains bromine derived from bromides as impurities, chlorine generated therefrom contains bromine as impurities.
When sodium hypochlorite is produced industrially, the above-described chlorine generated by electrolysis is used as a raw material. Therefore, in the disinfection of drinking water, which is the main purpose for sodium hypochlorite, and the like, it is necessary to reduce the concentration of bromic acid generated with bromine to a certain reference value or below.
As a method for reducing the bromine concentration in chlorine to or below a reference value set currently in Japan, a method of washing chlorine having a bromine concentration of not more than 2500 ppm with water containing substantially no bromine, in an amount of one time or more by weight with respect to chlorine, has been disclosed (e.g. JP 59 (1984)-92903 A). With this method, the bromine removal achieved by this method is at a level achieving the purpose with respect to the current reference value of the bromic acid concentration in sodium hypochlorite used in the disinfection of drinking water, etc. However, to achieve the purpose, a large amount of water is necessary for washing chlorine that contains bromine, which results in a large amount of chlorine water containing bromine as a by-product. Therefore, there arises a problem of disposing the by-produced chlorine water. Further, to preparing for the future demand for further reducing the amount of bromic acid remaining in sodium hypochlorite to be used in the disinfection of drinking water, a further technical innovation for reducing bromine contained in a chlorine gas is needed.
On the other hand, in the case where an organic chlorine compound is produced using a chlorine gas that contains bromine, an organic bromine compound generated with bromine as impurities is by-produced. This organic bromine compound has a property of being decomposed more easily by heat or light as compared with chlorine compounds of the same halogen group. Therefore, the organic bromine compound causes a chlorine compound as a product to degrade or discolor, causes an apparatus to be corroded, and functions as a catalyst poison with respect to a catalyst. Thus, bromine in chlorine contaminates a product formed from chlorine, and decreases the reactivity of an intermediate product formed from chlorine, thereby affecting the production of a final product. Recently, in the production of an organic chlorine compound product in particular, such chlorine having a smaller content of bromine as impurities is needed, and a new technique for reducing bromine contained in a chlorine gas is needed.